There are numerous circumstances where it is prudent to provide a releasable connection or coupling between the lower end of a running string of pipe or tubing and a tool suspended thereon that allows the running string to be recovered in the event that the tool gets stuck. For example when a downhole drilling motor is run on coiled tubing to drill a bore that is deviated substantially from the vertical, the possibility that the motor and its bit may get stuck is greatly enhanced. A release of the lower end of the coiled tubing is made more problematical by the fact that the tubing is continuous throughout its length, and thus has no joints that can be unthreaded. Moreover it can not be turned because it is wound on a large storage reel at the surface. Of course it will be recognized that a downhole tool is considered stuck when the operator pulls up on the running string to the allowable limit but the tool does not move upward, even though a greater pull might have dislodged the tool.
Various releasable devices of the type described above have been proposed, but have certain shortcomings. Some require that a ball be circulated down into a seat so that pressure can be applied to shear pins and trigger a release. However it is fairly common to employ an armored electric cable in the running string to provide a telemetry link with a downhole sensor package, so that a ball can not be used. Passage of the ball through the tubing with internal wireline is not always assured. Other devices have used a tension sleeve instead of shear pins, however the limitation of not being able to apply pulling force to the stuck tool once the sleeve fails remains as a significant drawback.
An object of the present invention is to provide a new and improved releasable connection apparatus that obviates the problems and shortcomings with prior devices.
Another object of the present invention is to provide a new and improved, releasable connection apparatus that operates to allow high pulling forces to be applied to a stuck well tool after the release mechanism has been triggered by tension in the running string.